Allow Element Call to be started without audio / video interface (#924)

* config: add feature in `config.json`

* groupCall: adjust connection state in feed if allowCallWithoutVideoAndAudio

* matrix-js-sdk: update version for allowCallWithoutVideoAndAudio

- I modified the SDK so that mute unmute work without media and check device permission inside the SDK
- allowCallWithoutVideoAndAudio is only checked at one point outside the SDK

* docu: add join group call without media docu in READMe

---------

Co-authored-by: Robin Townsend <robin@robin.town>
Co-authored-by: Šimon Brandner <simon.bra.ag@gmail.com>

src/config/ConfigOptions.ts

@@ -44,6 +44,14 @@ export interface ConfigOptions {
       server_name: string;
     };
   };
+
+  /**
+   * Allow to join a group calls without audio and video.
+   * TEMPORARY: Is a feature that's not proved and experimental
+   */
+  features?: {
+    feature_group_calls_without_video_and_audio: boolean;
+  };
 }
 
 // Overrides members from ConfigOptions that are always provided by the